This invention relates to the removal of solutes from aqueous solvent with reverse osmosis membranes. More particularly, the invention concerns the substantial removal of boron solute by operating the membrane at a relatively high pH.
As is well known to those skilled in the art, wastewater may contain a wide variety of undesirable components which restrict its use or disposal. As time passes, standards regulating the content of wastewater disposed into natural bodies of water or injected underground have become more and more strict. One industrial wastewater which may be produced in large quantities is oilfield produced water, produced concurrently with hydrocarbon production.
Produced water contains a wide variety of components depending upon its origin and the particular underground reservoir. Typically, a substantial portion of produced water is recovered including formation water and injected water. Formation water is that water which is naturally present in the oil or gas reservoir. It may amount to as little as 1% by volume at the beginning of production, but typically increases during the life of the well. Injected water is water which has been injected into the underground reservoir to enhance hydrocarbon recovery. This may be done by water flooding, surfactant flooding, steam flooding or other enhanced oil recovery processes. In such enhanced oil recovery processes, it is not unusually to have produced water comprising 90-95% of the produced fluids. Although the oil may have considerable value, produced water is a problem, requiring substantial cost and effort for disposal. In locations such as California, standards have become so strict that in many cases, the produced water cannot be disposed of by reinjection or by discharge into a local river, even though the produced water has less of a certain solute than it did before the oil company obtained the water from the same river.
Most produced water contains high concentrations of various water-soluble salts typified by those present in sea water. Most formation water is a high salt concentration brine. Frequently, the water that is injected into a formation in an enhanced oil recovery process is also brine. In most areas of the world, fresh water is more costly and not as abundant as brine. And even when a fresh water is injected into an underground hydrocarbon reservoir such as in steam flooding, the water may pick up substantial solutes during its passage through the underground reservoir.
Produced water also commonly contains immiscible hydrocarbons, dissolved hydrocarbons of all kinds including such carcinogens as benzene, toluene and xylene, dissolved water-soluble organic electrolytes such as fatty acids, carboxylic acids and phenols, and numerous other elements and compounds.
In California, boron poses a particular solute problem in produced waters. Boron, contained in sea water in a typical concentration of about 4.6 mg/L, is easily tolerated by humans at similar concentrations (as witnessed by drinking water standards). Citrus plants, however, are troubled with boron toxicity at low levels. The current California standard for irrigation water is a maximum of 0.75 mg/L. Oil companies have found it quite difficult and costly to reduce boron levels below 0.75 mg/L. This must be done even though the original source of the water prior to the water being obtained and injected by the oil company had a boron concentration higher than the standard. At present, this standard can only be met by the use of costly ion exchange resins. Membrane systems have failed to perform effectively due to fouling problems (frequently from dissolved hydrocarbons) and insufficiently high rejection rates for boron.
Numerous reverse osmosis membrane systems are used world-wide to desalinate seawater for potable water and irrigation purposes. In this use, boron solute concentrations are reduced. However, processing seawater with reverse osmosis membranes does not pose the same technical problems as processing oilfield produced waters containing solubilized oil, numerous other contaminates, and sometimes boron concentrations ranging as high as 20-50 mg/L.
Most of the world's boron is contained in seawater. But pure supplies of sodium borate exists in arid regions where inland seas have evaporated to dryness, especially in volcanic areas. Boron is frequently present in fresh water supplies from the same geological areas, such as California.
The Nalco Water Handbook, copyright 1988 by McGraw-Hill, notes that boron is present in water as nonionized boric acid, B(OH).sub.3. At a high pH over 10, most boron is present as the borate anion, B(OH).sub.4.sup.-. It is known that borate rejection by reverse osmosis membranes may increase with increasing feedwater pH. A graph of borate rejection by a DuPont membrane (B-9) plotting boron rejection versus feedwater pH was received from Infilco Degremont. The graph indicates borate rejections of 95% at a Ph of 10 or higher for the particular DuPont membrane.
Unfortunately, most reverse osmosis membranes are susceptible to degradation when operated at a feedwater pH of 10-11 or higher. In addition, the problems posed to membrane operation by produced waters containing numerous solutes including solubilized oil pose substantially different operating conditions than the seawater feed normally used with reverse osmosis membranes. Furthermore, calcium and magnesium scale poses a considerable fouling problem to reverse osmosis membranes at an operational pH of 10-11.
For reverse osmosis membranes, a charge liquid containing a more permeable and a less permeable component is maintained in contact under pressure with a non-porous separating layer. In a reversal of the cellular osmotic process, a portion of the charge liquid, predominately liquid, dissolves into the membrane and diffuses through with a decreased concentration of salts. Usually, a substantial portion of the solutes are left behind as the retentate.
One of the leading categories of reverse osmosis membranes are those composite membranes prepared from polyamides. As is the case with reverse osmosis membranes in general, the literature is voluminous with disclosures of different polyamide membranes. Polyamide membranes are described in U.S. Pat. Nos. 3,567,632; 3,600,350; 3,687,842; 3,696,031; 3,744,642; 3,878,109; 3,904,519; 3,948,823; 3,951,789; 3,951,815; 3,993,625; 4,005,012; 4,039,440; 4,259,183; 4,277,344; 4,812,238; 4,859,384; 4,888,116; 4,960,517; 4,964,998; and numerous others.
Various chemical processes have been developed wherein the pH of the aqueous liquid is raised or lowered for better reaction or separation purposes. An example of this is disclosed in U.S. Pat. No. 4,818,410 wherein water is treated by acidifying the fluid to a pH of 6 or lower with a strong acid to aid in separating water soluble organics from water to an oil phase.
U.S. Pat. No. 5,028,336 discloses a process of elevating pH to aid in the separation process with a polysulfone composite membrane. More particularly, the pH of an oilfield produced water in the range of 4-7 is raised to a pH of 7-9 to gain better rejection of water soluble organic electrolytes such as fatty acids, carboxylic acids and phenols.